This application is based on Japanese Patent Applications Nos. 2001-312962 and 2001-312963 both filed on Oct. 10, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a method and an apparatus which permit accurate measurement of temperature of an object body even in the case where the temperature of a wall such as a furnace wall surrounding the object body is different from that of the object body.
2. Discussion of Related Art
Temperature measuring methods of non-contact type are industrially useful and widely employed. To practice such non-contact type temperature measuring methods, there are known a radiation thermometer operable to effect a monochromatic temperature measurement, and a radiation thermometer operable to effect a dichroic temperature measruement. The former thermometer measures the temperature of an object body by comparing a radiant intensity value at one wavelength selected from an optical energy emitted from the object body with a reference value, which is a radiant intensity at the same wavelength of an optical energy emitted from a black body. Although this thermometer permits easy measurement of the temperature of the object body, it requires determination of the emissivity of the object body, and is not suitable for measuring temperature of an object body the emissivity of which changes. On the other hand, the latter thermometer can measure the temperature of an object body the emissivity of which is not unknown, since the temperature of the object body is determined on the basis of a ratio of radiant intensity values of two radiations having respective two different wavelengths selected from a radiant energy emitted from the object body, irrespective of the emissivity.
The radiation thermometers of non-contact type capable of monochromatic or dichroic temperature measurement or other non-contact type radiation thermometers may suffer from insufficient accuracy of the temperature measurement of the object body, due to a stray light noise undesirably included in the light radiated from the object body. More specifically described, the noise is a radiant energy of a stray light which is emitted toward a surface of the object body from the surroundings of the object body, e.g., inner wall surfaces of a furnace in which the object body is heated, and a heater or burner of the furnace. The stray light is reflected by the surface of the object body and incident on a photosensitive device of the radiation thermometer, so that the radiant intensity of the stray light is included in the detected radiant energy, namely, as a radiant energy as emitted from the object body itself. Thus, the detected temperature of the object body is adversely influenced by the radiant energy of the stray light. The degree of the adverse influence on the accuracy of measurement of the temperature of the object body increases with a rise in temperature of the inner wall surfaces of the furnace surrounding the object body, since the rise in the temperature of the surroundings causes an increase in the intensity of the radiant energy emitted from the inner wall surfaces of the furnace, as the stray light noise
JP-A-6-147989 discloses a radiation temperature measuring apparatus of non-contact type which is arranged to measure the temperature of an object body, by detecting a radiant energy emitted from the object body located within a furnace through an inspection opening of a water-cooled shielding plate while a radiant energy emitted from the wall surface is cut or shut off by the shielding plate. This conventional apparatus is effective when the temperature within the furnace is a relatively low near the room temperature. However, where the temperature in the furnace is relatively high, the water-cooled shielding plate cools down the object body, leading to deterioration of the temperature measurement accuracy. JP-A-6-258142 discloses another radiation temperature measuring apparatus of non-contact type. This apparatus uses two radiation thermometers for detecting radiant energies emitted from an object body and a furnace wall, respectively. The radiant energy emitted from the furnace wall, which is detected by one of the two thermometers, is multiplied by a known emissivity value of the object body, and the product is determined as a noise component derived from a stray light. The temperature of the object body is calculated based on a value of the radiant energy emitted from the object body as detected by the other thermometer minus the noise component from the radiant energy emitted from the object body as detected by the other thermometer, minus the noise component. This apparatus suffers from a drawback that the measurement accuracy is not sufficiently high when the temperature distribution within the furnace wall is uneven, since the temperature of the furnace wall is detected by the radiation thermometer at only one local portion of the furnace wall. In an electric furnace, for example, the temperature is considerably higher at a heat-generating portion than at the other portions of the furnace. Therefore, to employ a radiant energy emitted from one local portion of the entire wall surface as a radiant energy emitted from the furnace wall as a whole leads to deterioration in the accuracy of measuring the temperature of the object body.
In the above-described situation, the inventors have carried out various studies. In view of the fact that where an electric furnace is provided with an electric heater for heating an object body, a radiant energy emitted inwardly from the inner wall surface of the electric furnace increases in proportion to a drive voltage applied to the electric heater, the inventors have found that a stray light noise can be effectively removed from a detected radiant intensity of the object body according to a predetermined relationship between a radiant intensity of a stray light, which is emitted toward and reflected by the object body, and the drive voltage applied to the electric heater; upon measurement of the object body temperature, the actual radiant intensity of the stray light is obtained based on the known drive voltage actually applied to the heater and according to the above-indicated predetermined relationship. Then, the obtained radiant intensity of the stray light is removed from the radiant intensity of the radiation from the object body as detected by a suitable device, to obtain the intensity of a radiant energy which is emitted from the object body and which does not include the stray light noise.
The inventors have also found that the intensity of a radiant energy of a stray light as a noise can be easily removed from a detected intensity value of a radiant energy emitted from the object body, by providing a furnace with a shielding device operable between an open state for permitting the stray light to reach the object body and a closed state for inhibiting the stray light from reaching the object body, between the inner wall surface of the furnace and the object body. In this case, the shielding device is held in its closed state, when the intensity of the radiant energy emitted from the object body in the furnace is detected for measurement of the temperature of the object body, so that the shielding device functions to establish an even distribution of the intensity of the radiant energy of the stray light emitted from the furnace wall (provided with burners or an electric heater). That is, the intensity of the radiant energy of the stray light is determined on the basis of the temperature of the shielding device. The thus determined noise or the intensity of the radiant energy of the stray light is eliminated from the detected intensity of the radiant energy emitted from the object body, to obtain a true or net value of the radiant intensity of the radiation which is emitted from the object body and which does not include the astray light noise.
The present invention has been developed in view of the findings discussed above. It is a first object of the present invention to provide a method which permits highly accurate measurement of a surface temperature of an object body in a furnace. A second object of the invention is to provide an apparatus suitable for practicing the method.
The first object may be achieved according to a first aspect of this invention, which provides a method of measuring a temperature of an object body in an electric furnace, based on an intensity of a radiant energy emitted from the object body, the electric furnace being provided with an electric heater operable by application of a drive voltage thereto to heat the object body, the method comprising: a radiant-energy detecting step of detecting an intensity of a radiant energy emitted from the object body; a stray-light noise eliminating step of determining as a noise an intensity of a radiant energy of a stray light which is emitted from an inner wall surface of the electric furnace toward the object body and reflected by a surface of the object body, according to a predetermined relationship between the intensity of the radiant energy of the stray light and the drive voltage applied to the electric heater and based on an actually applied value of the drive voltage, and subtracting the intensity of the radiant energy of the stray light determined as the noise, from the detected intensity of the radiant energy emitted from the object body; and a temperature calculating step of calculating a temperature of the object body, based on the intensity of the radiant energy emitted from the object body from which the noise has been removed in the stray-light noise eliminating step.
According to this first aspect of the invention, the intensity of the radiant energy of the stray light emitted from the inner wall surface of the electric furnace toward the object body and reflected by the object body is determined based on the drive voltage actually applied to the heater and according to the predetermined relationship between the radiant energy intensity and the drive voltage, and the radiant energy intensity of the stray light determined as a noise is removed from the detected intensity of the radiant energy emitted from the object body, in the stray-light noise eliminating step. Then, in the temperature calculating step, the temperature of the object body is calculated based on the intensity of the radiant energy from which the noise has been removed. This arrangement assures highly accurate measurement of the temperature of the object body in the electric furnace.
One preferable form of the first aspect of the invention is applicable to a dichroic measurement of a distribution of a surface temperature of the object body in the electric furnace, by calculating a temperature of the object body at each picture element of its image, on the basis of a radiant intensity ratio at each pair of corresponding picture elements of a first and a second image which are obtained with respective first and second radiations having respective first and a second wavelengths and selected from a light emitted from the surface of the object body. In this preferred from of the method, the radiant-energy detecting step comprises: a first-wavelength radiant-energy detecting step of detecting a radiant intensity of said first radiation at said each picture element, said first-wavelength radiant-energy detecting step including selecting said first radiation having said first wavelength from the light emitted from the surface of said object body, by using a first filter which permits transmission therethrough of said first radiation having said first wavelength which is selected according to a radiant-intensity curve corresponding to a wavelength of a black body at a lower limit of a range of the temperature to be measured, and which is within a high radiant intensity range in which the radiant intensity is higher than a radiant intensity at a normal room temperature, said first filter permitting transmission therethrough of a radiation having a half width which is not larger than {fraction (1/20)} of said first wavelength; and a second-wavelength radiant-energy detecting step of detecting a radiant intensity of said second radiation at said each picture element, said second-wavelength radiant-energy detecting step including selecting said radiation having said second wavelength from the light emitted from the surface of said object body, by using a second filter which permits transmission therethrough of said second radiation having said second wavelength which is selected within said high radiant intensity range, such that said second wavelength is different from said first wavelength by a predetermined difference which is not larger than {fraction (1/12)} of said first wavelength and which is not smaller than a sum of a half width of said second wavelength, and wherein said stray-light noise eliminating step comprises determining an intensity of a radiant energy of said stray light at each picture element of each of the first and second images, and subtracting the determined intensity of the radiant energy of the stray light at each picture element of each of the first and second images, from the intensity of the radiant energy emitted from the object body at the corresponding picture element obtained in a corresponding one of said first-wavelength radiant-energy detecting step and said second-wavelength radiant-energy detecting step, so as to obtain intensities of the radiant energies of the first and the second radiation at each picture element from which the intensity of the radiant energy of the stray light has been removed and said temperature calculating step comprises calculating the temperature of the object body at said each picture element, by obtaining, at said each picture element, a ratio of the intensity of the radiant energy of the first radiation from which the intensity of the radiant energy of the stray light has been removed, to the intensity of the radiant energy of the second radiation from which the intensity of the radiant energy of the stray light has been removed.
In the present method, the intensity of the radiant energy of the stray light which is a light emitted from the inner wall surface of the electric furnace toward the object body and reflected by the surface of the object body and which is included in the intensity of the radiant energy of each of the first and second radiations is first determined at each picture element, according to the predetermined relationship between the drive voltage applied to the electric heater of the electric furnace and the intensity of the radiant energy of the stray light, and based on the actual value of the applied voltage. The intensity of the radiant energy of the stray light is then eliminated from the intensity of the radiant energy of each of the first and second radiations at each picture element, which has been detected as the intensity of the radiant energy of the first or second radiation emitted from the object body. Based on the thus obtained intensity, the temperature of the object body at each picture element is calculated. Accordingly, the temperature of the surface of the object body located inside the electric furnace can be obtained with high accuracy. Further, in the above preferred form of the method, the temperature of the object body at each picture element of its image is calculated on the basis of the radiant intensity ratio at each pair of mutually corresponding the picture elements of the first and second images obtained with the respective first and second radiations having the respective first and second wavelengths selected from the light emitted from the surface of the object body. To select the first radiation having the first wavelength from the light emitted from the surface of the object body, the present method uses the first filter which permits transmission therethrough of the first radiation having the first wavelength which is selected according to the radiant-intensity curve corresponding to the wavelength of the black body at the lower limit of the range of the temperature to be measured, which is within the high radiant-intensity range in which the radiant intensity is higher than the radiant intensity at the normal room temperature, and which has a half width which is not larger than {fraction (1/20)} of the first wavelength. The present method further uses the second filter which permits transmission therethrough of the second radiation having the second wavelength which is selected within the above-indicated high radiant-intensity range, such that the second wavelength is different from the first wavelength by a predetermined difference which is not larger than {fraction (1/12)} of the first wavelength and which is not smaller than a sum of the half width of the first wavelength and the half width of the second wavelength. Accordingly, optical signals having sufficiently high radiation intensities can be obtained, leading to an accordingly high S/N ratio. In addition, the first and second wavelengths are close to each other, so that the principle of measurement according to the present invention fully matches the principle of measurement by a dichroic thermometer, namely, fully meets a prerequisite that the dependency of the emissivity on the wavelength can be ignored for two radiations the wavelengths of which are close to each other, leading to approximation xcex51=xcex52. Thus, the present measuring method permits highly accurate measurement of the temperature distribution.
The second object may be achieved according to a second aspect of this invention, which provides an apparatus for measuring a temperature of an object body in an electric furnace, based on an intensity of a radiant energy emitted from the object body, the electric furnace being provided with an electric heater operable by application of a drive voltage thereto to heat the object body, the apparatus comprising: a radiant-energy detecting means for detecting an intensity of a radiant energy emitted from the object body; a stray-light noise eliminating means for determining as a noise an intensity of a radiant energy of a stray light which is emitted from an inner wall surface of the electric furnace toward the object body and reflected by a surface of the object body, according to a predetermined relationship between the intensity of the radiant energy of the stray light and the drive voltage applied to the electric heater, based on an actually applied value of the drive voltage, and subtracting the intensity of the radiant energy of the stray light determined as the noise, from the detected intensity of the radiant energy emitted from the object body; and a temperature calculating means for calculating a temperature of the object body, based on the intensity of the radiant energy emitted from the object body from which the noise has been removed by the stray-light noise eliminating means.
According to this second aspect of the invention, the intensity of the radiant energy of the stray light emitted from the inner wall surface of the electric furnace toward the object body and reflected by the object body is first determined based on the drive voltage actually applied to the electric heater according to the predetermined relationship between the radiant energy intensity and the drive voltage, and the intensity of the radiant energy of the stray light determined as a noise is removed from the detected intensity of the radiant energy emitted from the object body. Then, the temperature calculating means calculates the temperature of the object body, based on the intensity of the radiant energy from which the noise has been removed. This arrangement assures highly accurate measurement of the temperature of the object body in the electric furnace.
One preferable form of the second aspect of the invention is a dichroic measurement of a distribution of a surface temperature of the object body in the electric furnace, by calculating a temperature of the object body at each picture element of its image, on the basis of a radiant intensity ratio at each pair of corresponding picture elements of a first and a second image which are obtained with respective first and second radiations having respective first and second wavelengths and selected from a light emitted from the surface of the object body. In this preferred form of the apparatus, the radiant-energy detecting means comprises: first-wavelength radiant-energy detecting means for detecting a radiant intensity of the first radiation at the each picture element, the first-wavelength radiant-energy detecting means including selecting the first radiation having the first wavelength from the light emitted from the surface of the object body, by using a first filter which permits transmission therethrough of the first radiation having the first wavelength which is selected according to a radiant-intensity curve corresponding to a wavelength of a black body at a lower limit of a range of the temperature to be measured, and which is within a high radiant intensity range in which the radiant intensity is higher than a radiant intensity at a normal room temperature, the first filter permitting transmission therethrough of a radiation having a half width which is not larger than {fraction (1/20)} of the first wavelength; and a second-wavelength radiant-energy detecting means for detecting a radiant intensity of the second radiation at the each picture element, the second-wavelength radiant-energy detecting means including selecting the radiation having the second wavelength from the light emitted from the surface of the object body, by using a second filter which permits transmission therethrough of the second radiation having the second wavelength which is selected within the high radiant intensity range, such that the second wavelength is different from the first wavelength by a predetermined difference which is not larger than {fraction (1/12)} of the first wavelength and which is not smaller than a sum of a half width of the second wavelength, and wherein the stray-light noise eliminating means comprises determining an intensity of a radiant energy of the stray light at each picture element of each of the first and second images, and subtracting the determined intensity of the radiant energy of the stray light at each picture element of each of the first and second images, from the intensity of the radiant energy emitted from the object body at the corresponding picture element obtained in a corresponding one of the first-wavelength radiant-energy detecting means and the second-wavelength radiant-energy detecting means, so as to obtain intensities of the radiant energies of the first and the second radiation at each picture element from which the intensity of the radiant energy of the stray light has been removed and the temperature calculating means comprises calculating the temperature of the object body at the each picture element, by obtaining, at the each picture element, a ratio of the intensity of the radiant energy of the first radiation from which the intensity of the radiant energy of the stray light has been removed, to the intensity of the radiant energy of the second radiation from which the intensity of the radiant energy of the stray light has been removed.
In the present apparatus, the intensity of the radiant energy of the stray light, which is a light emitted from the inner wall surface of the electric furnace toward the object body and reflected by the surface of the object body and which is included in the intensity of the radiant energy of each of the first and second radiations is first determined at each picture element, according to the predetermined relationship between the drive voltage applied to the electric heater of the electric furnace and the intensity of the radiant energy of the stray light, and based on the actual value of the applied voltage. The intensity of the radiant energy of the stray light at each picture element is then eliminated from the intensity of the radiant energy of each of the first and second radiations at each picture element, which has been detected as the intensity of the radiant energy of the first or second radiation emitted from the object body. Based on the thus obtained intensity, the temperature of the object body at each picture element is calculated. Accordingly, the temperature of the surface of the object body located inside the electric furnace can be obtained with high accuracy. Further, in the above preferred form of the apparatus, the temperature of the object body at each picture element of its image is calculated on the basis of the radiant intensity ratio at each pair of mutually corresponding two picture elements of the first and second images and obtained with the respective first and second radiations having the respective first and second wavelengths selected from the light emitted from the surface of the object body. To select the first radiation having the first wavelength from the light emitted from the surface of the object body, the present apparatus uses the first filter which permits transmission therethrough of the first radiation having the first wavelength which is selected according to the radiant-intensity curve corresponding to the wavelength of the black body at the lower limit of the range of the temperature to be measured, which is within the high radiant-intensity range in which the radiant intensity is higher than the radiant intensity at the normal room temperature, and which has a half width which is not larger than {fraction (1/20)} of the first wavelength. The present apparatus further uses the second filter which permits transmission therethrough of the second radiation having the second wavelength which is selected within the above-indicated high radiant-intensity range, such that the second wavelength is different from the first wavelength by a predetermined difference which is not larger than {fraction (1/12)} of the first wavelength and which is not smaller than a sum of the half width of the first wavelength and the half width of the second wavelength. Accordingly, optical signals having sufficiently high radiation intensities can be obtained, leading to an accordingly high S/N ratio of the apparatus. In addition, the first and second wavelengths are close to each other, so that the principle of measurement according to the present invention fully matches the principle of measurement by a dichroic thermometer, namely, fully meets a prerequisite that the dependency of the emissivity on the wavelength can be ignored for two radiations the wavelengths of which are close to each other, leading to approximation xcex51=xcex52. Thus, the present measuring apparatus permits highly accurate measurement of the temperature distribution.
The first object body may be achieved according to a third aspect of this invention, which provides a method of measuring a temperature of an object body in a heating furnace, based on an intensity of a radiant energy emitted from the object body, the method comprising: a heating step of heating the object body while a shielding device disposed between the object body and an inner wall surface of the heating furnace and operable between an open state for permitting a stray light to be emitted from an inner wall surface of the heating furnace and a closed state for inhibiting the stray light from reaching the object body, is held in the open state; a radiant-energy detecting step of detecting an intensity of a radiant energy emitted from the object body while the shielding device is held in the closed state; and a temperature calculating step of calculating a temperature of the object body, based on the intensity of the radiant energy emitted from the object body detected in the radiant-energy detecting step.
In this method, the object body is heated while the shielding device located between the object body and the furnace wall is held in the open state in the heating step, and then the intensity of the radiant energy emitted from the object body is detected while the shielding device is held in the closed state, in the radiant-energy detecting step. In the following temperature calculating step, the temperature of the object body is obtained, based on the thus detected intensity of the radiant energy emitted from the object body. According to this method, the intensity of the radiant energy of the stray light emitted from the furnace wall toward the object body is evenly distributed in the presence of the shielding device held in its closed state, during the detection of the intensity of the radiant energy emitted from the object body. An intensity of the noise (radiant energy of the stray light) is determined according to a predetermined relationship between the temperature of the shielding device and the determined intensity of the radiant energy of the stray light, and based on the intensity of the radiant energy of the stray light. Accordingly, the stray light noise can be easily removed from the intensity of the radiant energy detected as the intensity of the radiant energy emitted from the object body, enhancing the accuracy of the measurement of surface temperature of the object body.
One preferable form of the third aspect of the invention is applicable to a dichroic measurement of a distribution of a surface temperature of the object body in the electric furnace, by calculating a temperature of the object body at each picture element of its image, on the basis of a radiant intensity ratio at each pair of corresponding picture elements of a first and a second image which are obtained with respective first and second radiations having respective first and a second wavelengths and selected from a light emitted from the surface of the object body. In this preferred from of the method, the heating step comprises heating the object body while the shielding device disposed between the object body and an inner wall surface of the heating furnace is held in the open state; the radiant-energy detecting step comprises: a first-wavelength radiant-energy detecting step of detecting a radiant intensity of the first radiation at the each picture element while the shielding device is held in the closed state, the first-wavelength radiant-energy detecting step including selecting the first radiation having the first wavelength from the light emitted from the surface of the object body, by using a first filter which permits transmission therethrough of the first radiation having the first wavelength which is selected according to a radiant-intensity curve corresponding to a wavelength of a black body at a lower limit of a range of the temperature to be measured, and which is within a high radiant intensity range in which the radiant intensity is higher than a radiant intensity at a normal room temperature, the first filter permitting transmission therethrough of a radiation having a half width which is not larger than {fraction (1/20)} of the first wavelength; and a second-wavelength radiant-energy detecting step of detecting a radiant intensity of the second radiation at the each picture element while the shielding device is held in the closed state, the second-wavelength radiant-energy detecting step including selecting the radiation having the second wavelength from the light emitted from the surface of the object body, by using a second filter which permits transmission therethrough of the second radiation having the second wavelength which is selected within the high radiant intensity range, such that the second wavelength is different from the first wavelength by a predetermined difference which is not larger than {fraction (1/12)} of the first wavelength and which is not smaller than a sum of a half width of the second wavelength; and wherein the temperature calculating step comprises calculating the temperature of the object body at the each picture element, by obtaining, at the each picture element, a ratio of the intensity of the radiant energy of the first radiation detected in the first-wavelength radiant-energy detecting step, to the intensity of the radiant energy of the second radiation detected in the second-wavelength radiant-energy detecting step.
In this preferred from of the method, the shielding device is held open in the heating step for heating the object body, and then brought into its closed state and held in this closed state in the first-wavelength radiant-energy detecting step and the second-wavelength radiant-energy detecting step for detecting the intensities of the radiant energies of the first and second radiations having the respective first and second wavelengths which are selected from the light emitted from the object body. In the following temperature calculating step, the temperature of the object body is calculated at each picture element, based on the thus obtained intensities of the radiant energies of the first and second radiations, that is, a ratio of the intensity of the radiant energy of the first radiation to the intensity of the radiant energy of the second radiation. According to this method, the stray light noise (the intensity of the radiant energy of the stray light) emitted from the furnace wall toward the object body and reflected by the surface of the object body, which noise is included in the intensity of the radiant energy detected as the intensity of the radiant energy emitted from the object body, is evenly distributed by the shielding device, and the intensity of the stray light noise is determined based on the temperature of the shielding device. Then, the noise or the intensity of the radiant energy of the stray light is eliminated from the detected intensity of the radiant energy emitted from the object body. This method thus enhances the accuracy of measuring the surface temperature of the object body. Further, in the above preferred form of method, the temperature of the object body at each picture element of its image is calculated on the basis of the radiant intensity ratio at each pair of mutually corresponding two picture elements of the first and second images and obtained with the respective first and second radiations having the respective first and second wavelengths selected from the light emitted from the surface of the object body. To select the first radiation having the first wavelength from the light emitted from the surface of the object body, the present method uses the first filter which permits transmission therethrough of the first radiation having the first wavelength which is selected according to the radiant-intensity curve corresponding to the wavelength of the black body at the lower limit of the range of the temperature to be measured, which is within the high radiant-intensity range in which the radiant intensity is higher than the radiant intensity at the normal room temperature, and which has a half width which is not larger than {fraction (1/20)} of the first wavelength. The present invention further uses the second filter which permits transmission therethrough of the second radiation having the second wavelength which is selected within the above-indicated high radiant-intensity range, such that the second wavelength is different from the first wavelength by a predetermined difference which is not larger than {fraction (1/12)} of the first wavelength and which is not smaller than a sum of the half width of the first wavelength and the half width of the second wavelength. Accordingly, optical signals having sufficiently high radiation intensities can be obtained, leading to an accordingly high S/N ratio. In addition, the first and second wavelengths are close to each other, so that the principle of measurement according to the present invention fully matches the principle of measurement by a dichroic thermometer, namely, fully meets a prerequisite that the dependency of the emissivity on the wavelength can be ignored for two radiations the wavelengths of which are close to each other, leading to approximation xcex51=xcex52. Thus, the present measuring method permits highly accurate measurement of the temperature distribution.
The second object may be achieved according to a fourth aspect of this invention, which provides an apparatus for measuring a temperature of an object body in a heating furnace, based on an intensity of a radiant energy emitted from the object body, the apparatus comprising: a shielding device provided between the object body and an inner wall surface of the heating furnace and operable between an open state for permitting a stray light to be emitted from the inner wall surface to reach the object body and a closed state for inhibiting the stray light from reaching the object body; a radiant-energy detecting means for detecting an intensity of a radiant energy emitted from the object body while the shielding device is held in the closed state; and a temperature calculating means for calculating a temperature of the object body, based on the intensity of the radiant energy emitted from the object body detected by the radiant-energy detecting means.
According to the fourth aspect of the invention, the intensity of the radiant energy emitted from the object body is detected by the radiant-energy detecting means while the shielding device disposed between the object body and the furnace wall, and the temperature of the object body is calculated by the temperature calculating means, based on the thus detected intensity of the radiant energy. According to this arrangement, the intensity of the noise (the radiant energy of the stray light) emitted from the furnace wall toward the object body is evenly distributed in the presence of the shielding device held in its closed state during the detection of the intensity of the radiant energy emitted from the object body. The intensity of the radiant energy of the stray light can be determined according to a predetermined relationship between a temperature of the shielding device and the intensity of the radiant energy of the stray light, and based on the thus determined value of the intensity of the radiant energy of the stray light. Accordingly, the stray light noise can be easily removed from the intensity of the radiant energy detected as the intensity of the radiant energy emitted from the object body, enhancing the accuracy of the measurement of the surface temperature of the object body.
One preferable form of the fourth aspect of the invention is a dichroic measurement of a distribution of a surface temperature of the object body in the electric furnace, by calculating a temperature of the object body at each picture element of its image, on the basis of a radiant intensity ratio at each pair of corresponding picture elements of a first and a second image which are obtained with respective first and second radiations having respective first and second wavelengths and selected from a light emitted from the surface of the object body. In this preferred form of the apparatus, the radiant-energy detecting means comprises: a first-wavelength radiant-energy detecting means for detecting a radiant intensity of the first radiation at the each picture element while the shielding device is held in the closed state, the first-wavelength radiant-energy detecting means including selecting the first radiation having the first wavelength from the light emitted from the surface of the object body, by using a first filter which permits transmission therethrough of the first radiation having the first wavelength which is selected according to a radiant-intensity curve corresponding to a wavelength of a black body at a lower limit of a range of the temperature to be measured, and which is within a high radiant intensity range in which the radiant intensity is higher than a radiant intensity at a normal room temperature, the first filter permitting transmission therethrough of a radiation having a half width which is not larger than {fraction (1/20)} of the first wavelength; and a second-wavelength radiant-energy detecting means for detecting a radiant intensity of the second radiation at the each picture element while the shielding device is held in the closed state, the second-wavelength radiant-energy detecting means including selecting the radiation having the second wavelength from the light emitted from the surface of the object body, by using a second filter which permits transmission therethrough of the second radiation having the second wavelength which is selected within the high radiant intensity range, such that the second wavelength is different from the first wavelength by a predetermined difference which is not larger than {fraction (1/12)} of the first wavelength and which is not smaller than a sum of a half width of the second wavelength, and wherein the temperature calculating means comprises calculating the temperature of the object body at the each picture element, by obtaining, at the each picture element, a ratio of the intensity of the radiant energy of the first radiation detected by the first-wavelength radiant-energy detecting means, to the intensity of the radiant energy of the second radiation detected by the second-wavelength radiant-energy detecting means.
In this apparatus, the shielding device is held open by the heating means during heating of the object body, and then brought into the closed state and held in the closed state while the first-wavelength radiant-energy detecting means and the second-wavelength radiant-energy detecting means respectively detect the intensities of the radiant energies of the first and second radiations having the respective first and second wavelengths selected from the light emitted from the object body. Next, the temperature calculating means calculates the temperature of the object body at each picture element, based on the thus detected intensities of the radiant energies of the first and second radiations, that is, a ratio of the intensity of the radiant energy of the first radiation to the intensity of the radiant energy of the second radiation. According to this arrangement, the noise or intensity of the radiant energy of the stray light emitted from the furnace wall toward the object body and reflected by the surface of the object body, which noise is included in the intensity of the radiant energy detected as the intensity of the radiant energy emitted from the object body, is evenly distributed by the shielding device, and the intensity of the stray light noise is determined based on the temperature of the shielding device. Then, the noise or the intensity of the radian energy of the stray light is eliminated from the detected intensity of the radiant energy emitted from the object body. This arrangement thus enhances the accuracy of measuring the surface temperature of the object body. Further, in the above apparatus, the temperature of the object body at each picture element of its image is calculated on the basis of the radiant intensity ratio at each pair of mutually corresponding two picture elements of the first and second images obtained with the respective first and second radiations having the respective first and second wavelengths selected from the light emitted from the surface of the object body. To select the first radiation having the first wavelength from the light emitted from the surface of the object body, the present apparatus uses the first filter which permits transmission therethrough of the first radiation having the first wavelength which is selected according to the radiant-intensity curve corresponding to the wavelength of the black body at the lower limit of the range of the temperature to be measured, which is within the high radiant-intensity range in which the radiant intensity is higher than the radiant intensity at the normal room temperature, and which has a half width of which is not larger than {fraction (1/20)} of the first wavelength. The present invention further uses the second filter which permits transmission therethrough of the second radiation having the second wavelength which is selected within the above-indicated high radiant-intensity range, such that the second wavelength is different from the first wavelength by a predetermined difference which is not larger than {fraction (1/12)} of the first wavelength and which is not smaller than a sum of the half width of the first wavelength and the half width of the second wavelength. Accordingly, optical signals having sufficiently high radiation intensities can be obtained, leading to an accordingly high S/N ratio of the apparatus. In addition, the first and second wavelengths are close to each other, so that the principle of measurement according to the present invention fully matches the principle of measurement by a dichroic thermometer, namely, fully meets a prerequisite that the dependency of the e-missivity on the wavelength can be ignored for two radiations the wavelengths of which are close to each other, leading to approximation xcex51=xcex52. Thus, the present measuring apparatus permits highly accurate measurement of the temperature distribution.
In each of the preferred forms of the methods and apparatuses according to the first to fourth aspects of the present invention, the first and second filters are preferably arranged such that the first filter permits transmission therethrough of the radiation having the half width which is not larger than {fraction (1/20)} of the first wavelength, while the second filter permits transmission therethrough of the radiation having the half width which is not larger than {fraction (1/20)} of the second wavelength. According to this arrangement, the radiations having the first and second wavelengths are considered to exhibit a sufficiently high degree of monochromatism. Therefore, the present invention meets the prerequisite for the principle of measurement by a dichroic thermometer, resulting in an improved accuracy of measurement of the temperature distribution.
The first and second filters used in the temperature measuring methods and apparatuses according to the first to fourth aspects of the invention are preferably arranged such that the first and second filters have transmittance values whose difference is not higher than 30%. This arrangement assures high sensitivity and S/N ratio, even for one of the two radiations of the first and second wavelengths which has a lower luminance value, permitting accurate measurement of the temperature distribution.